Filter systems comprising one or more filter units, are conventionally used in many industrial facilities to remove contaminant solids from process liquid streams. A conventional filter unit comprises a filter housing enclosing one or more filter elements. A filter element typically strains or screens solids from the liquid stream. The filter element typically has a selected pore size and is intended to remove solids of larger than pore size from the liquid stream. The filtered-out solids tend to gradually accumulate, or cake, on the input surface of the filter element. Thus, the liquid flow rate through the filter tends to decrease, and the liquid pressure across the filter element tends to increase, with the growth of the cake and hence with the length of time of filter use.
The accumulated solids cake on the filter element eventually grows enough to block or occlude the filter element enough to impair filtering efficiency. Thus, eventually action is required to unblock the filter element to restore filtering efficiency.
Various prior art techniques have been used to unblock the filter element.
One prior technique is to simply replace the blocked filter element with a new filter element. However, this is expensive in terms of downtime, while the filter assembly is off-line during element replacement, and in terms of new filter element cost.
In another prior technique, the blocked filter element is removed, manually regenerated, or regenerated in an external regenerating liquid tank, and replaced. However, this is expensive in terms of filter unit downtime, and in terms of extra labor costs for manual regenerating.
Another prior art technique is backwashing. In backwashing, the blocked filter element is switched off-line, and liquid flow through the filter element in a reverse direction dislodges the solids from the inlet side of the filter element and carries them out of the filter housing. However, backwashing may leave some of the contaminating solids on the filter element and results in downtime for the filter assembly.
More important, one backwash of one filter element may require a large quantity of liquid (e.g. 30 to 40 gallons to backwash a single 10 gallon filter housing). Backwashing may be required as frequently as every 15 minutes. Often, backwashing requires use of filtered process liquid or a special backwashing liquid which may be very expensive compared for example to water. Also, the waste stream of liquid from backwashing must be discarded in an environmentally permissible manner or treated to restore it for reuse, both of which involve additional problems, process steps and expense.
U.S. Pat. No. 5,059,331 to Goyal suggests another prior art technique in which backwashing and ultrasonic vibration of the filter unit are performed simultaneously. However, the Goyal prior art technique combines disadvantages of the above-discussed prior techniques, including the above-discussed disadvantages of backwashing.
Accordingly, the objects and purposes of the present invention include providing a filtration method and apparatus which avoid disadvantages of the above-discussed prior art techniques.
Further objects and purposes of the invention will be understood by persons familiar with apparatus of this general type by referring to the following description taken in conjunction with the accompanying drawings.